Traditionally, electrical power has been produced by large, geographically separated facilities and transmitted as high-voltage alternating current to other locations. These large Power generation facilities are connected through a network, sometimes referred to as a power grid such that power produced by locations having excess power generation capacity can be diverted to areas where loads may be particularly large at any given time. In the proximity of loads, the voltage is generally reduced in stages and further distributed until the location of various loads is reached. The high voltage used for transmission over long distances allows currents and resistive losses to be reduced while using cables of reduced conductor material content. Use of alternating current allows the reduction of voltage by the use of transformers. Alternating current can also be directly used by many common and familiar loads such as household appliances, pumps using electric motors and the like.
However, many familiar loads are principally based on electronic circuits which are rapidly increasing in number and power requirements and the proportion of the load of many other devices that is presented by electronics (e.g. processor controlled appliances) is also increasing rapidly. Many new devices such as electrically powered vehicles are also being introduced. Additionally, environmental concerns have encouraged the development of local power generation and/or storage systems in many locations to serve local “islands” or groups of customers where power distribution can be provided as either AC or direct current (DC). Power storage must generally be provided with DC power. Therefore, the need for conversion to tailor the conventionally supplied power from AC networks into desired but different AC and DC power is proliferating rapidly at the present time.
Power converters are, by their nature, non-linear and their dynamic behaviors are coupled with those of the load from which they receive power or the source providing power through them. As a consequence, many power electronics systems will require control in order to provide a regulated output. However, provision of such regulation causes additional phenomena that have not been previously observed or considered to be of importance, including but not limited to issues of stability.
Specifically, a power converter under regulated output control exhibits negative incremental impedance characteristics at its input. That is, in the case of converters regulating voltage (to a different form of that of the source), the current consumed by them is inversely proportional to voltage variations of the source in order to maintain a constant power flow to the load. This is the inverse behavior of resistive loads, whose current consumption is directly proportional to voltage variations of the source. Consequently, the small-signal response at a given operating point, corresponding to the linearization of the converter at such point, presents negative phase. As is well recognized in the art, negative impedance can result in instabilities and possibly oscillatory behavior of the circuit with detrimental effects to the system where they operate.
While the extensive power grid can tolerate many of these behaviors since the effect of converter behavior is small compared to the size of the system, such behaviors cannot be tolerated by smaller systems which have their own, relatively small capacity power source and are not connected to the effectively infinite power grid. Examples of such smaller systems are aircraft, water-borne vessels, hybrid electric vehicles and small power plants (e.g. wind turbines or solar collector farms) serving individuals or small “islands” of customers. Other examples of circumstances where unstable behavior may occur are instances where electrical loads are connected through equipment such as frequency changers (AC/AC converters), AC/DC converters and other types of hardware. Vehicular systems also operate at higher line frequencies than the line frequencies traditionally used for power distribution and present other phenomena and challenges in regard to control.
Systems which can potentially exhibit unstable behaviors are becoming prevalent due to proliferation of systems such as are discussed above and, further, by shifting functions previously performed mechanically or hydraulically to electrically powered functions. Accordingly, it is imperative that potential instabilities be made predictable and avoided in the design of such systems. Therefore, stability of electrical systems has been a subject of substantial interest and study in recent years; yielding some solutions for DC systems such as DC/DC converters. However, there are issues not seen in DC systems which are present in AC systems and multi-phase AC/DC systems, in particular, which are referred to as multi-variable systems. While some progress has been made in regard to determining stability or forbidden operating conditions of multi-phase AC/DC systems, the analysis has been extremely complex and burdensome and has, in general, led to excessively conservative designs and operating parameters.